Modification of a plant trait through genetic engineering depends upon the insertion into the plant genome of a polynucleotide construct containing the gene of interest, operably linked to a promoter that is functional in the transgenic plant. Within a plant genome, any single gene is, in general, operably linked to a promoter that will determine when and where, within the plant tissues and organs, the gene should be expressed. Therefore if one wants to express a gene of interest in specific tissues or organs within a transgenic plant and in a temporally regulated manner, tissue-preferred promoters must be used. On the other hand, expression in all plant tissues throughout the plant's life cycle could by achieved by using constitutive promoters.
In a number of situations the expression of particular genes in particular tissues or organs confers a specific phenotype of interest to the plant. For example, if one wants to improve the nutritional quality of cereal seeds, a gene that confers such phenotype using seed-specific promoters is inserted, rather than using constitutive promoters that would allow the gene to be expressed in all plant tissues causing, in some cases, undesirable phenotypes. In another example, if one wants to increase the amount of cellulose in the developing vascular tissues of a forest tree, one would introduce into the plant genome a xylem- and/or cambium-preferred promoter operably linked to a heterologous gene encoding an enzyme involved in cellulose metabolism such that more cellulose molecules could be produced in the developing plant xylem. In another example, the desired phenotype could be obtained by inhibiting the expression of an endogenous gene within a specific plant tissue. This could be done by introducing a construct comprising a tissue-preferred promoter operably linked to a polynucleotide that would inhibit the expression of the endogenous gene, either by anti-sense hybridization or by RNA silencing (Matzke (ed.) et al. (2000) Plant Gene Silencing Kluwer Academic Publishers).
Thus far, the production of genetically engineering plants expressing useful and/or desirable traits requires the availability of promoters that permit the gene or genes of interest to be expressed in a tissue- and timing-specific manner. Thus, isolation and characterization of tissue-preferred, particularly cambium/xylem-preferred, promoters that can serve as regulatory regions for expression of heterologous nucleotide sequences of interest in a tissue-preferred manner is essential for the genetic engineering of plants exhibiting particular traits.